1. Field of the Invention
This invention relates to ballast circuits for gas discharge lamps, and, more particularly, to a ballast circuit for three series-connected, low pressure gas discharge lamps.
2. Description of the Prior Art
In designing lighting systems, a determination must be made of the amount of light necessary for a task or function to be performed within the area to be lighted. The type and number of lamps must be compatible with the quantity and quality of light required. In some applications, three gas discharge lamps are employed in a three lamp fixture to provide the required lighting. Gas discharge lamps, for example, fluorescent lamps, require a ballast circuit to control the electrical power supplied to each of the lamps. A difficulty with the prior art three lamp fixtures is that no energy efficient three lamp ballast is available to supply all three lamps from a single ballast.
A prior art three lamp ballast is shown schematically in FIG. 3. A transformer 110 having a primary winding 112 connected to a power line providing 120-volt 60 Hertz a.c. power and a secondary winding 114 supplies a.c. power to power capacitor 116. A discharge resistor 118 is connected across capacitor 116 as a safety measure. A starting capacitor 120 with a parallel-connected discharge resistor 122 is connected to the output side 119 of capacitor 116. Preheating windings 124, 126, 128 and 130 are connected to electrodes disposed within the respective ends of conventional 40-watt lamps 132, 134 and 136. The starting capacitor 120 is connected to one terminal between electrodes of lamps 132 and 134.
The prior art circuit of FIG. 3 operates as follows. Electrical power is applied to the input terminals of transformer primary winding 112. Preheat current is supplied to the preheat windings 124, 126, 128 and 130. Starting power is applied at terminals 119 and 138 to series-connected lamps 132, 134 and 136 and to starting capacitor 120. Capacitor 120 allows current to substantially bypass lamp 132. The voltage and current applied to lamps 134 and 136 causes a glow due to partial ionization of the discharge gas in the lamps. The partial ionization and the large voltage from the power supply circuit causes ignition of lamps 134 and 136 which drops the voltage across the lamps 134 and 136 and imposes a starting voltage on lamp 132. The open circuit voltage across terminals 119 and 138 was approximately 445 volts. The lamp voltage was in the range of 100 volts for each lamp. Therefore, the ratio of open circuit voltage to total lamp voltage was approximately 1.5. The efficacy of the system was calculated to be approximately 60.1 lumens per watt (LPW). This overall system efficacy is not as high as that of conventional two lamp systems. Further, the high open circuit voltage represents some hazard to personnel or surrounding equipment, and is not the voltage level normally used in power distribution. In order to limit the hazard of high voltage, recessed contact or interrupting lampholders, which disable the lamp circuit whenever a lamp is removed, are used. This adds cost and complexity to the lamp system.
Another prior art approach to three lamp fixtures typically employs a two lamp ballast and a relatively inefficient one lamp ballast in the same fixture to provide two distinct power supply circuits, one supplying a pair of the lamps and the other supplying the remaining lamp of the three lamp set. This two lamp ballast and one lamp ballast combination approach circumvents the need for recessed contact or interrupting lampholders which are substantially more expensive to buy and install than the standard bipin lamp base and lampholders. Each ballast exhibits its operating characteristics including losses from its own secondary winding or isolated transformer in the power supply circuit. By requiring two ballasts, the overall efficacy in lumens of light output per watt of energy input is diminished by the losses associated with using two ballasts. One reason for employing two ballasts in a single fixture was to limit the power supply voltage to a pre-established level. The prior art recognized a required minimum voltage of 395 volts RMS to start three lamps connected in series. This high voltage requirement creates difficulty in meeting safety standards required for gas discharge lighting systems. Therefore, the prior art approach has been to limit each ballast circuit to one or two lamps in order to limit the required lamp starting voltage.